The present invention relates to electrostatic precipitators, and, more particularly, to circuits and methods for energizing such precipitators.
Electrostatic precipitators are electrical devices employed to remove particulate matter from a gaseous stream directed between oppositely charged precipitator electrodes. Precipitators are used in a number of industrial applications including chemical plants, and, more particularly, including electric power plants and other potential sources of particulate pollution. Recently, electrostatic precipitators have been much more frequently employed than in the past, because of the increased needs and desires for particulate removal from gases vented to the atmosphere. It should also be appreciated that the electrostatic precipitators of primary concern herein are high energy devices typically consuming several tens of kilowatts of electrical energy. Accordingly, proper precipitator energization is important, not only with respect to particle collection efficiency, but also with respect to economy and reliability of precipitator operation.
Many precipitator designs have been in the past. However, each basically operates upon fairly well-established principles. Precipitators generally include a pair of conductive electrodes. Typically, one of the electrodes comprises two parallel plane metallic sheets which are typically spaced about nine inches apart. The sheets are typically operated at ground potential. Additionally, a planar array of wires, connected electrically together and disposed midway between and parallel to the conductive sheets, comprises the second electrode. This planar wire array electrode is maintained at high electric potential. While it is possible to ground the wire electrodes and apply high potential voltage to the electrode sheets, this mode of operation is typically avoided for safety reasons. A number of the parallel plane electrodes are assembled in a housing which defines a plurality of parallel gas flow passages through the volume between the precipitator electrodes. These passages are also defined, at least in part, by the structure and arrangement of plate electrodes. In general, commercial precipitators employ a plurality of plate and wire grid electrode pairs. The area of a typical section of a precipitator may, in fact, possess up to about 30,000 square feet of plate electrode area. Naturally, such a configuration exhibits a certain amount of electrical capacitance between the wire and plate electrodes. The capacitance of a typical precipitator section is on the order of 0.05 to 0.15 microfarads. While the operation of such precipitators appears to be relatively simple, there are several phenomena which occur, which can limit precipitator particle collection efficiency. Different methods of energizing precipitators significantly affect the amount of electrical energy and power expended in removing a given fraction of particles from a gas stream in a given precipitator and with a given type of particle. It should be pointed out that precipitators generally operate at peak voltages of between about 40,000 and 80,000 volts and each section may draw current of about 1.5 amperes. It is thus easily seen that precipitator power levels of 80 kilowatts are not uncommon. Therefore, electrical efficiency is a significant economic factor in plants employing electrostatic precipitators for the removal of particulate matter. Furthermore, for continuous plant operation, reliability of the precipitator and precipitator energizing components is very important.
In normal operation, particulate matter in the gas to be treated acquires a negative charge as the result of induced ionization effects occurring principally in the vicinity of the precipitator cathode wires. The charged dust particles are then attracted to the precipitator anode plates where a layer of anode dust accumulates. As this dust layer accumulates, an increasingly thick dust layer is formed on the plate (that is, sheet) electrode. In the situation in which the particulate matter comprises significant portions of high resistivity material, the voltage drop across the high resistivity dust layer near the anode plates also acts to reduce the voltage drop between the cathode and the dust layer and can reduce particle charging and collection. Moreover, a high electric field can be created within the dust layer such that there is a tendency for an efficiency-robbing back-corona discharge to occur within the dust layer. These high resistivity dust layers can exhibit back-corona phenomena in which ions are actually emitted from the dust layer toward the cathode wires. Consequently, this back-corona phenomena acts to reduce particle charging and collection. Even though the dust layer may be periodically removed by means of vibrating, rapping, or otherwise flexing the anode plates, there is still an efficiency reduction concomitant with the formation of this highly-resistive layer. For the case of high resistivity dust, it is desirable to limit the current through the dust layer so as to preclude this back-corona phenomenon. One solution that has been proposed for this problem is to operate the precipitator near the corona on-set voltage. However, this method results in current inhomogeneities, loss of efficiency and presents control difficulties. Accordingly, efficient but yet effective and economical ways of energizing precipitators are highly desirable, particularly for the collection of dust particles exhibiting high resistivity, that is, a resistivity of about 10.sup.31 11 ohm-cm, or higher. For example, such dusts are created in the burning of low sulfur coal used by the electric utility industry.
The design and operation of electrical circuits for precipitator energization has taken many different paths in hope of arriving at a method of precipitator operation which is efficient, reliable, controllable and relatively inexpensive to implement. Furthermore, there has not been broad agreement amongst the practitioners in this art with respect to optimal precipitator energization methods. The proliferation of directions which various practitioners have undertaken is exemplified in the discussion below in which specific articles and patents are considered. However, it will be appreciated that the various pulse energization methods considered by other practitioners in the art, generally fall into one of two categories: broad pulse powering and narrow pulse powering. It should also be appreciated that electrical circuitry for precipitator energization is typically limited to producing two basic pulse waveforms, namely, the sinusoidal pulse and the rectangular pulse. In those prior art precipitator designs which principally rely on the production of sinusoidally-shaped pulses, it will generally be appreciated that the cost of circuit components which are capable of such operation is generally relatively inexpensive since rapid switching problems do not occur. However, the nature of the sinusoidal pulse is inherently limited. For example, broad pulse powering methods employing sinusoidal pulses must, of necessity, employ sinusoidal waveforms (typically rectified waveforms) which have a relatively long period because of the broad width nature of the pulse. However, such sinusoidal waveforms, with their large periods, exhibit slow rise and fall times so that the time duration of peak precipitator energization is relatively small. Accordingly, the amount of power which can be efficiently applied to dust collection in this time period is significantly reduced.
As an alternative to sinusoidal pulse energization methods, rectangular pulses exhibiting relatively sharp rise and fall times and relatively flat peak levels may also be effectively employed in precipitator energization. However, it must be born in mind that, in this case, "precipitator energization" means the rapid switching of extremely high voltage and current levels. While it is possible to produce and employ such rectangular pulses, it is nonetheless often impractical to do so without incurring a large component cost for switching elements which are required to perform the rapid switching function. Furthermore, even if an economic investment is made in such high voltage switching equipment, its long-term reliability in field operations is often a significant problem because of the great electrical stresses which are of necessity applied to such high voltage, high speed, high energy switching components. Accordingly, rectangular pulses employed for electrostatic precipitator energization exhibit significant limitations.
Several experimenters in the art of electrostatic precipitators have proposed various means for electrically enhancing the collection efficiency of an electrostatic precipitator over that obtainable with conventional rectified a.c. powering. These methods have involved the use of electrical pulses superimposed on a steady reference voltage and fall into the two general categories recited above: narrow pulse powering and broad pulse powering. Narrow pulse powering systems have been described, for example, by Research-Cottrell, Inc. in an article in their technical bulletin, copyrighted in 1979, titled "Technological Development of Pulse-Energization". In the narrow pulse powering scheme disclosed, pulses of very short duration (approximately 1 microsecond or less) are applied together with a conventional rectified 60 Hertz reference voltage. However, there are several significant problems associated with narrow pulse energization schemes. First among these problems is the cost of the circuit components required to perform the high speed switching functions. Additionally, energy recovery in narrow pulse powering schemes is very difficult and is seldom, if ever, achieved. A factor in the inability of the narrow pulse powering systems to recover energy supplied in one part of the cycle is the fact that a significant portion of the energy supplied to the system of wires and plates may in fact be radiated as electromagnetic waves. This not only results in the loss of a portion of the usable energy, but it also can significantly result in the production of radio frequency interference and associated EMI problems. Another significant problem associated with narrow pulse powering systems is the reliability of the circuit components, as pointed out above. Therefore, it is seen that narrow pulse energization systems exhibit significant limitations as precipitator energization systems.
Another circuit which discloses pulse powering of electrostatic precipitators using pulses having a duration of approximately 70 microseconds, is disclosed in an article by Helge H. Petersen titled "New Trends in Electrostatic Precipitation: Wide Duct Spacing, Precharging, Pulse Energization" appearing in Volume IA-17, No. 5 of the IEEE Transactions on Industry Applications in the September/October, 1981 issued on pages 496-501. In the operation of the circuit disclosed in the Petersen article, the d.c. component of the energizing waveform is maintained just below the corona on-set level so that the pulse provides a major portion of the energy for particle collection. Control of precipitator current is accomplished by varying pulse frequency. To supply the level of pulse power which is desired, Petersen teaches that it is generally necessary to apply the pulse voltage to the precipitator for a relatively long period of time, if it is to perform any significant amount of work on the particle. Furthermore, the voltage pulse generated by the circuit illustrated in the Petersen article exhibits a voltage undershoot characteristic which is not in itself a major problem, but under circumstances in which the pulse frequency is increased, the undershoot increases in magnitude and the resultant voltage applied to the precipitator becomes an essentially continuous sinusoidal wave symmetrically superimposed on the d.c. reference voltage. As a result, the pulse voltage subtracts from the reference voltage every half-cycle and is less effective than a unidirectional pulse in enhancing particle charging and collection processes, particularly when the reference voltage is close to the corona on-set voltage. Accordingly, there exists a need for an improved electrostatic precipitator pulsing system which addresses these difficulties.
Precipitator systems exhibiting pulse durations between about 0.2 and 2 milliseconds have been generally described as broad pulse powered systems. The optimal or near optimal waveform for pulsing a precipitator would assure that high current and voltage are applied simultaneously for the maximum duration which is commensurate with avoiding back-corona discharge. Sustained operation at these high voltages results in maximum particle charge, enhanced particle migration and a uniform current density. Broad pulse powered systems having many of these characteristics are described, for example, in the article titled "High Voltage Thyristors Used In Precipitators" published in Control Engineering, 1981, written by Jerry F. Shoup and Thomas Lugar. In the described system, two high voltage valves are required, each comprising a plurality of thyristors connected in series along with voltage equalizing components to obtain the desired voltage rating. These valves unfortunately can suffer from reliability problems because of the voltage stresses which are applied to them. Furthermore, these valves are also relatively expensive.
In U.S. Pat. No. 3,915,672, issued Oct. 28, 1975 to Gaylord W. Penney, there is disclosed an electrostatic precipitator energization circuit; however, the disclosed circuit requires switching of high voltages which can significantly impact circuit costs and reliability. Moreover, the precipitator disclosed in the Penney patent is directed specifically to a precipitator employing a three-electrode structure.
In general, broad pulse powering methods have required that the pulse waveform exhibit a flat waveshape in which the peak level is maintained for a time sufficient to supply the desired amount of power to the precipitator to enable it to perform its collection function. In the case of high resistivity dusts, particle collection rates can be increased substantially for those waveforms exhibiting short rise and fall times relative to the pulse duration. Additional improvement is noted by the present inventors with the use of rapidly pulsed electric fields in which improvement is attributed to the fact that back-corona in layer dust is apparently inhibited by the rapid fluctuations, whereas it is less inhibited in a relatively slowly time varying electric field. The present inventors attribute this phenomenon to the effective presence of a relatively large RC time constant. This time constant arises from a model of the dust layer as a parallel combination of a resistor and a capacitor. Certainly, this layer exhibits both of these effects.
In U.S. Pat. No. 4,052,177, issued Oct. 4, 1977 to Leif Kide, a circuit is disclosed which returns some of the stored capacitive energy on the precipitator to a d.c. storage capacitor to increase the overall electrical efficiency of the circuit. However, the pulse powering methods disclosed by Kide are essentially the same as discussed above in the Petersen article. The rise and fall times of the pulse waveshapes are determined by the half sinusoidal shape of the pulse generated by the circuits of such systems.
Additional background material on the powering of electrostatic precipitators may be found in the article "A Pulse Method for Supplying High Voltage Power for Electrostatic Precipitation" by H. J. White, appearing in the November 1952 issue of the IEEE Transactions, on page 326 thereof.
In short, it is seen that one of the significant problems in the powering of electrostatic precipitators is the necessity of providing control of precipitator current to prevent back-corona while maintaining high efficiency. These have generally been considered to be contradictory control and design goals. Economical and controllable energizing methods which provide simultaneous application of voltage, particle charging and current are nonetheless desired. It further appears that the peak voltage of the pulse should be sustained for a controllable duration to provide the necessary precipitation energy. However, the transformer-coupled systems of Kide and Petersen generate a sinusoidal rather than flat top pulse so that the peak voltage exceeds that used by the broad pulse generated by the systems of Lugar and Shoup or Penney for a given level of pulse energy. As a result, back-corona conditions are more likely to occur. However, because of the expense, difficulty and reliability associated with the high voltage electrical switching components in broad pulse powering systems, another method and means would be highly desirable whereby the benefits of broad pulse powering could be achieved with improved reliability and lower costs. Furthermore, it is desired that these improvements occur without loss of control of precipitator power and current levels and without the concomitant production of undesirable and wasteful levels of radio frequency radiation. Applicants'invention, as discussed below, is seen to attain these objectives.
Because the width of the pulse in a broad system is typically chosen so that it is on the order of, or slightly shorter than, ion transit time within the precipitator, substantially all of the charge injected into the precipitator during the pulse resides in the gas volume between the electrodes when the pulse is terminated. This large space charge can act to shield the corona electrodes and can reduce the electric field in the vicinity of this electrode. Consequently, the injected current is a maximum at the beginning pulse and decreases with time. This phenomenon is true for broad pulse systems in general. However, the ramped pulse burst precipitator energization method disclosed herein significantly alleviates such problems.
From the above discussion relating to the energization of electrostatic precipitators, it should be appreciated that there is wide disagreement amongst practitioners in the art as to what constitutes an optimal energization method. Furthermore, there is an even greater array of opinions attempting to explain, on a theoretical basis, the effectiveness of one method over another. This is a direct result of the fact that, alothough precipitators operate on fairly well-understood fundamental principles, nonetheless there are a number of critical secondary effects and control variables that can be employed in the design of precipitator energizing circuits. However, the pulse burst energization scheme disclosed by the present inventors has not hithertofore been disclosed or proposed as a solution to the array of conflicting problems associated with precipitator energization, particularly when high resistivity dusts are to be collected. None-theless, the method of the present invention overcomes a number of the problems associated with precipitator energization, as discussed above, and in particular, the method of the present invention exhibits high reliability, low cost, low levels of electromagnetic radiation and the capability for energy recovery.